Measuring systems for determining the gamma dose rate in routine operation can be based essentially on purely counting systems that employ a calibration factor to convert the counting rate into a dose rate. The technical challenge faced by manufacturers of measuring instruments is to adapt the energy- and angle-dependence of the measuring system to the measured parameter to be mapped, and to do so in such a way that this measured parameter can be described as precisely as possible by optimizing the arrangement of the active components and of the passive absorbers. The introduction of spectrometric systems such as, for instance, NaI or high-resolution Ge detectors, has opened up new possibilities for determining the gamma dose rate. Ascertaining photon spectra entails advantages during the analysis of radiation fields. For instance, it is relatively easy to determine several dose parameters such as, for example, the ambient equivalent dose or, assuming the radiation geometries PA (parallel from the back), AP (parallel from the front), ISO (uniformly from everywhere), LAT (laterally), ROT (rotating), etc., also organ doses or the effective dose.
Up until now, commercially available spectrometric detection systems have only employed approximation methods in order to ascertain the photon spectrum and the dose spectrum. The distributions measured using a detector and a multichannel analyzer are only approximately real photon spectra and they have to be corrected with respect to the events in which only partial-energy deposits of the incident photons occur. In the case of high-resolution detectors, there are deconvolution methods with which photon spectra can be ascertained from measured pulse height distributions at a high energy resolution.
The dose spectrum can be ascertained on the basis of the photon spectrum by using dose conversion factors and the integral dose can be determined by summing up the dose spectrum. The deconvolution method uses detector-specific response functions that can be computed, for instance, with Monte Carlo programs. The energy resolution of the response functions corresponds at best to that of the detector, but in actual practice, it is approximately five to ten times the energy resolution of the detector. The deconvolution method is based on a matrix inversion. The examples cited in the literature are realized for a wide energy range up to approximately 2 MeV or 3 MeV; therefore, it is necessary to work with a large matrix and numerous operations for the matrix inversion.
This method calls for a great deal of memory space and takes up CPU time in order to ascertain the photon spectra. If the method is repeated frequently, the use of resources, that is to say, the computation capabilities and energy consumption, is needlessly high which, in the case of smaller integrated measuring instruments, translates into increased time consumption and memory requirements.
A method for nuclide-specific exposure estimation was described in a presentation of the GSI [Gesellschaft für Schwerionenforschung mbH—Institute for Heavy Ion Research] on spectrometric photon dosimetry on Apr. 26, 2005. The local dose is measured in accordance with Article 39 of the German Radiation-Protection Ordinance [Strahlenschutzverordnung—StrlSchV]. The measuring of the local dose is carried out to augment or replace the determination of the personal dose according to Article 41 of StrlSchV. The method consists of employing a detector, HPGe or NaI, for example, to measure the pulse height distribution Mi. With a Monte Carlo program, for instance, EGS4, the response function is calculated, e.g. isotropically or in parallel, as a function of the geometry, yielding the response matrix Rij. On this basis, the energy distribution of the photons, the photon spectrum Φj, is determined via the mapping equationMi=ΣRijΦj.In this context, a summation is carried out of j=1 to nmax. (In the mapping equation, Mi is the measured distribution.) In the next step, the organ doses or the effective dose are obtained from the energy distribution of the photons using conversion factors such as, for instance, the ambient equivalent dose H* (10). An example of response functions of the ambient radiation in a laboratory, the spectral kerma distribution (KERMA=kinetic energy released in matter), and the example of the activation on an accelerator, among others, are presented.
In 2002, G. Fehrenbacher et al. presented a paper titled “Analyse der Aktivierung von Beschleunigerstrukturen und der damit verbundenen möglichen Strahlungsexposition durch Gammastrahlung” [Analysis of the activation of accelerator structures and of the associated possible radiation exposure to gamma radiation]. This paper provides examples of gamma spectra that were measured at the beam hole of the heavy-ion synchrotron of the German Institute for Heavy Ion Research (Gesellschaft für Schwerionenforschung mbH—GSI) after a period of radiation with deuterium ions on structures with elevated radiation losses. On the basis of the pulse height distribution measured employing a portable HPGe detector, the spectral photon flux density and the dose spectrum were ascertained by means of deconvolution. The response functions needed for the deconvolution were determined with the EGS4 simulation program. The dose rates determined on the basis of the spectra are compared to the measured values obtained employing a Geiger-Müller counter. The fraction of unscattered radiation in the total dose rate is estimated on the basis of an example.
In Health Physics, February 1998, Vol. 74, No. 2, A. Clouvas et al. present the essay titled “Conversion of in-situ gamma ray spectra”. The suggestion is made to convert an in-situ γ-ray spectrum into a photon flux energy distribution, the conversion being based on the Monte-Carlo method. The spectrum was measured with a portable Ge detector. The spectrum is first freed of the partial-absorption and cosmic-ray events so as to leave only the events that are associated with the full absorption of the gamma radiation. Based on the remaining spectrum, the efficiency curve of the full-energy deposits of the detector, ascertained by means of calibrated point sources and Monte-Carlo simulations, is employed and the photon flux energy distribution is then derived. The events that have to do with the particle absorption in the detector are calculated by means of the Monte-Carlo simulation for various incident photon energies and angles.
The deconvolution method, which has not yet become common practice in the technical world, would be too laborious and resource-consuming for the mere determination of the dose rate if the work is carried out at full resolution of the detector with the full number of channels of the multichannel analyzer. For routine cases, where there is no need for radionuclide association, the computation and memory requirements invested are needlessly high.
Up until now, there are only instruments that are used exclusively as spectrometers and whose software component was developed separately for the above-mentioned method.